Why is an timestamp offset for TS Muxings applied by default?

This is done to ensure correct audio visual sync, and to avoid negative DTS (DecodingTimeStamp), which break some decoders. Some of them apply a different start value for the audio and video PTS (PresentationTimeStamp). This can cause slight audio visual sync issues. Instead, a slight offset is applied to all streams so that no negative DTS values occur and all streams are aligned.

Why is that?

Streams contain a lot of details, including timestamps, so a decoder knows how to handle the content properly. The DTS decides when a frame has to be decoded, while the PTS describes when a frame has to be presented. This difference becomes important when using B-frames, which are frames that can have references to frames in the past, but also to frames in the future. Given that, there will be frames in the future, which a decoder needs to decode first in order to use them as reference. So they will have a smaller DTS than their PTS. As DTS values can not be negative, muxers introduce a short offset for the PTS values to accommodate for the fact that DTS can be smaller than the PTS values.
As the PTS values are used to align the presentation time of different streams (audio and video) to each other, all streams need to start with the same PTS offset, to not introduce any sync issue.

How it is solved

The “best practise” way in order prevent these audio/video sync problems when processing such content is to set a PTS offset of 10 seconds. This is the default value used or TS muxings in our service as well as of other vendors. In our service it can be adjusted when creating a TS muxing as well. Please see the startOffset property when creating a TS Muxing in the API reference)

What about subtitles?

WebVTT is a common way to provide subtitles for video content. It contains time windows when a certain subtitle has to be shown, and is therefore synchronized to the video content.
The timing information in the WebVTT track gets converted into a timestamp information by the player. However, the calculated timestamp would be 10 seconds behind the actual point in time the subtitle would have to be shown. While many HTML5 players can compensate that, native players don't do that necessarily.
In order to provide them with this information, the X-TIMESTAMP-MAP can be used for that, as described in the HLS specification.

So, if WebVTT already contains this information, it would start like that:

1WEBVTT

2X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000

The value of MPEGTS actually describes an PTS offset of 10 seconds. The default timescale for MPEG-TS is 90,000. In order to achieve an offset of 10 seconds you multiply it by the timescale, which results in 900,000.